No phenomenon of the life of the soul is so wonderful and so variously interpreted as consciousness. The most contradictory views are current to-day, as they were two thousand years ago, not only with regard to the nature of this psychic function and its relation to the body, but even as to its diffusion in the organic world and its origin and development. It is more responsible than any other psychic faculty for the erroneous idea of an "immaterial soul" and the belief in "personal immortality"; many of the gravest errors that still dominate even our modern civilization may be traced to it. Hence it is that I have entitled consciousness "the central mystery of psychology"; it is the strong citadel of all mystic and dualistic errors, before whose ramparts the best-equipped efforts of reason threaten to miscarry. This fact would suffice of itself to induce us to make a special critical study of consciousness from our monistic point of view. We shall see that consciousness is simply a natural phenomenon like any other psychic quality, and that it is subject to the law of substance like all other natural phenomena. Even as to the elementary idea of consciousness, its contents and extension, the views of the most distinguished philosophers and scientists are widely divergent. Perhaps the meaning of consciousness is best conceived as an _internal perception_, and compared with the action of _a mirror_. As its two chief departments we distinguish objective and subjective consciousness--consciousness of the world, the non-ego, and of the ego. By far the greater part of our conscious activity, as Schopenhauer justly remarked, belongs to the consciousness of the outer world, or the non-ego: this _world-consciousness_ embraces all possible phenomena of the outer world which are in any sense accessible to our minds. Much more contracted is the sphere of _self-consciousness_, the internal mirror of all our own psychic activity, all our presentations, sensations, and volitions.